Warming, insect pests, and water stress combine to reduce tree growth in the city

Cities are getting warmer. This is due in part to global climate change. The more important factor for now, though, is the urban heat island effect, local warming in cities caused by sidewalks, asphalt, and reduced tree cover. In short, areas with less shade and more impervious surface area absorb more heat and trap it – releasing it more slowly than other areas after the sun goes down. These urban heat islands have a number of consequences for trees. Trees planted in warmer areas within cities often have more insect pests, and these same trees grow less than trees in cooler areas nearby.

As part of my graduate thesis, I studied how urban warming and insect pests affect street trees, and a mystery began to nag at me. Some trees in the hottest areas within cities were covered in insect pests and still looked vigorous, while other trees with the same pest densities withered. I thought this pattern might be because trees have access to different amounts of resources—depending on where and how they are planted—and that trees with more resources might be better able to tolerate or compensate for insect pests. This is a simple idea that had not been tested, and its implications are of critical importance. People are healthier when we have access to greenery, and urban trees can only provide ecosystem services – such as reducing air pollution and trapping carbon — if they can photosynthesize and grow.

To approach this mystery, I did an experiment across the city of Raleigh, NC and another, more controlled experiment in the laboratory. In the city, I chose forty trees across a gradient of urban temperatures. At each site, one tree was sprayed for pest insects, and another was left unsprayed. In the lab, I did essentially the same experiment but also manipulated the amount of water trees had access to. Water is often scarce for urban trees because they have limited rooting space and are often planted in compacted soils.

I found that trees in hotter areas of the city were more water stressed. In the lab, water stress reduced tree growth, but warming and insect pests acting alone did not affect tree growth. However, trees that were water stressed, hot, and infested with insect pests grew the least. What this means is that trees can compensate for insect pests and warming if they have enough water. If trees don’t have enough water, warming and insect pests combine with water stress to reduce tree growth. Together, these experiments showed that some urban trees may be more affected by pests than others because they have access to less water.

Street trees in the hottest areas of the city were more water stressed. Photo: EK Meineke

These findings provide information on how we might be able to better manage urban forests so that cities are green and vibrant in the future:

1) Revisit how we assess insect pests. To keep cities green, entomologists and land managers periodically count insect pests on plants. However, counting insects might not be the best way to understand how pests affect plants, because a tree’s ability to make up for what insects remove depends on the resources they have access to. Urban forests may benefit from a more holistic process for assessing when insects are actually acting as pests and, on the other hand, when we expect that trees are able to compensate for their damage. This framework could reduce how much we have to treat trees with pesticides, which are major threats to biodiversity.

2) Make sure trees have enough water. As trees get hotter due to climate change and urbanization, the amount of water they have access to will become even more important for tree growth than it is now. Our results suggest that, for many trees, providing adequate rooting space and watering during heat waves may be key strategies for reducing the effects of warming, water stress, and insect pests as they intensify with local and global warming.

3) Plant for the future. When planting trees in cities, we may want to consider drought-resistant plants and genotypes to maximize the number of large shade trees in future urban forests. One area of future research is to determine if these drought-resistant plants are also better able to compensate for insect pests.